Isocyanatoaryl sulphochlorides are known. They are prepared (according to German Pat. No. 947,159) by the phosgenation of aromatic amino sulphonic acids Disadvantages of this process are the need to use high boiling solvents such as dichlorobenzene and, in particular, nitrobenzene, and the relative insolubility of the amino sulphonic acids used as starting materials in these solvents even at the high reaction temperatures used. The reaction, therefore, takes a long time and the yield of isocyanatoaryl sulphochlorides obtained is unsatisfactorily low. This applies particularly to the preparation of diisocyanatoaryl sulphochlorides. One diisocyanatoaryl sulphochloride was obtained in only 46% of the theoretical yield in Example 3 of the abovementioned patent. Due to these disadvantages, the process has not become commercially important.
It is also known to prepare simple aryl sulphochlorides by the sulphonation of aromatic compounds, e.g. with chlorosulphonic acid or with mixtures of chlorosulphonic acid and sulphuryl chloride (see e.g. Houben-Weyl 4th Edition, Volume 9, pages 563-585). It is also known to prepare sulphochlorides from free sulphonic acids or their salts with acid chlorides. These processes, which are known in the literature, have not been applied or have been applied with only negative results in the preparation of isocyanato sulphochlorides from isocyanates. This is due to a variety of causes.
First, sulphochlorination with chlorosulphonic acid requires an excess of the latter reactant. Thus, the product must be worked up in the presence of water which attacks the isocyanate groups. Secondly, isocyanatosulphonic acids have only become known very recently and have proved to be extremely reactive and very difficult to handle. Thirdly, it is known that sulphonic acid groups undergo uncontrolled and unpredictable reactions with isocyanates at elevated temperatures with the elimination of CO.sub.2. Thus, when sulphochloride is prepared under the usual conditions, other reactions must be expected to take place at the same time. Examples of these other reactions are sulphone and anhydride formation and polymerization reactions of the isocyanate group.
Isocyanatoaryl sulphochlorides containing at least two isocyanate groups would be highly interesting substances with a considerable potential for application. In particular, the corresponding derivatives of 2,4-diisocyanatotoluene, which is commercially the most important diisocyanate in polyurethane chemistry, would have considerable potential. The reasons for this interest are
(a) these compounds should have a very low vapor pressure compared to unmodified diisocyanate; PA0 (b) these compounds should have a higher polarity than the unmodified diisocyanate; and PA0 (c) their products of hydrolysis (aromatic diamine sulphonic acids) are largely nontoxic.
It was, therefore, an object of the present invention to provide 2,4-diisocyanatotoluene having sulphochloride substituents and an easy commercial process for their preparation.
It was surprisingly found that despite the difficulties to be expected, as mentioned above, this object could be achieved with the process according to the present invention described below. The compounds according to the present invention described below were also found to be valuable starting materials for the preparation of polyisocyanate polyaddition products. They combine not only the advantages mentioned above but also impart improved flame resistance to the polyaddition products prepared from them.